1. Field of the Invention
The present invention relates to a method and communication device utilized in a wireless communication system, and more particularly, to a method and communication device utilized in the wireless communication system for generating a buffer status of a packet data convergence protocol (PDCP) layer.
2. Description of the Prior Art
As today's applications for electronic systems grow at ever-increasing rates, the demand for better communications performance is never ceasing. Standards for various technologies such as the 3rd Generation Partnership Project (3GPP) High-Speed Packet Access (HSPA) and Long Term Evolution (LTE) work towards creating more efficient communication systems.
For example, dynamic resource allocation for uplink (UL) transmissions has been introduced into the medium access control (MAC) layer standard, for communicating between a user equipment (UE) such as a mobile station or a mobile handset, and an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) including evolved base stations (eNBs).
Dynamic resource allocation utilizes radio resources more efficiently, where radio resources are allocated only when the UE has data to transmit. The E-UTRAN schedules UE transmission based on UE buffer status. The UE buffer status reporting has a great impact on efficiency of overall radio source usage arranged by E-UTRAN. Reporting more data volume than actual data volume in the buffer makes unnecessary uplink transmission and consumes UE power. Reporting less data volume than actual data volume in the buffer makes the UE inevitably request for more resources to be allocated and wait for the uplink resource allocation, and this causes a delay before the UE data can be transmitted, slowing down the overall communication between the UE and the E-UTRAN.
In striving for faster and better communications and efficient usage of radio resource in our wireless communications systems, the unnecessary uplink resource allocation and uplink transmission delay should be alleviated.
A PDCP (packet data convergence protocol) layer of the UE provides data transfer services (e.g. radio bearer transfer) to an RRC (radio resource control) layer and data protocol layers (e.g. Internet Protocol), and exchanges packets of a UM (unacknowledged mode) and AM (acknowledged mode) mode of an RLC (radio link control) layer.
The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data in the UL buffers of the UE. The Buffer Status Report is defined as a MAC (Medium Access Control) Control Element including all data that is available for transmission in the RLC layer and in the PDCP layer. A Buffer Size field in the Buffer Status Report MAC Control Element identifies the total amount of data available across all logical channels of a logical channel group after a MAC PDU has been built. The amount of data is indicated in number of bytes.
For the purpose of MAC buffer status reporting, the UE shall consider the following as data available for transmission in the PDCP layer:
the SDUs for which no PDU has been submitted to lower layers:
the SDU itself, if the SDU has not yet been processed by PDCP, or
the control or data PDU if the SDU has been processed by PDCP.
In addition, for radio bearers that are mapped on the RLC AM, if the PDCP entity has previously received an indication that a handover has occurred, the UE shall also consider the following as data available for transmission in the PDCP layer:
For SDUs for which a corresponding PDU has only been submitted to lower layers before PDCP has received an indication that a handover has occurred, and for which the successful delivery has not been confirmed by lower layers or by a PDCP status report:
the SDU, if it has not yet been processed by PDCP, or
the PDU (data only) once it has been processed by PDCP.
When RRC connection re-establishment occurs, the RLC is re-established, and the RLC entity discards all RLC SDUs and RLC AMD (AM data) PDUs at the transmitting side. In other words, all data buffered in RLC for transmission are deleted. When a data radio bearer (DRB) mapped to RLC AM is resumed after the RRC connection re-establishment, the buffer status report only includes data below in the PDCP entity of the DRB.
For SDUs for which no PDU has been submitted to lower layers:
the SDU itself, if the SDU has not yet been processed by PDCP, or
the PDU (control or data) if the SDU has been processed by PDCP.
As can be seen from the above, the buffer status report does not include the PDCP PDUs that have been submitted to RLC but have not been confirmed by RLC for successful transmission before RRC connection re-establishment. Those PDCP PDUs may need to be retransmitted are not reported. As a result, the E-UTRAN cannot know a correct buffer data volume of the UE and therefore cannot precisely allocate UL grant.
For example, a PDCP entity has submitted all data of a DRB to a RLC AM entity and no data has been sent by RLC AM successfully due to radio link failure. When the DRB is resumed after the RRC connection re-establishment, the buffer status report includes no data in buffer since all data in the RLC buffer has been cleared. In this situation, the E-UTRAN does not allocate any UL grant for transmission to the UE according to the buffer status report.
The same abovementioned situation also occurs in a RLC indication in which the maximum number of retransmissions has been reached or a handover failure is seen.